1. Field of the Invention
This invention relates to a semiconductor photodetector available for an optical measuring instrument and an optical communication system and, more particularly, to an avalanche photo-diode (hereinafter abbreviated as APD) for producing a sharp pulse signal.
2. Description of the Related Art
The APD per se serves as an amplifier in an optical measuring instrument and an optical communication system, and germanium (Ge) and indium-gallium-arsenide (InGaAs) are available for the APDs at wavelength of 1.3 .mu.m and 1.55 .mu.m.
FIG. 1 illustrates a typical example of the APD formed of germanium, and comprises a substrate 1, a photo-incident region 2 defined in a surface portion of the substrate 1, a guard ring 3 of partially overlapped with the outer periphery of the photo-incident region 2, a transparent passivation film 4 covering the surface of the substrate 1 and electrodes 5, 6 held in contact with the photo-incident region 2 and the rear surface of the substrate 1. The substrate 1 is formed of n-type Ge, and boron is implanted into the surface portion for the photo-incident region 2. The guard ring 3 is formed through a thermal diffusion of zinc, and the transparent passivation film 4 is deposited by using a chemical vapor deposition.
In operation, the p-n junction between the substrate 1 and the photo-incident region 2 is reverse biased so as to extend a depletion layer therefrom, and incident light produces electron-hole pairs in the depletion layer. The carriers thus produced in the presence of the photon are accelerated in the electric field created in the depletion layer, and electric current flows between the electrodes 5 and 6.
Turning to FIG. 2 of the drawings, another prior art APD in the InP-InGaAs-InGaAsP system is illustrated. This prior art APD is fabricated on a substrate of heavily doped n-type InP 11, and comprises a buffer layer 12, a photo-absorbing layer 13, an intermediate layer 14, a multiplication layer 15, a window layer 16 with a photo-incident region 161, a transparent passivation film 17 and electrodes 18 and 19 respectively held in contact with the photo-incident region 161 and the reverse surface of the substrate 11.
The buffer layer 12 to the window layer 16 are epitaxially grown on the substrate 11 through a phase epitaxy, and are formed of n-type InP doped at 1.times.10.sup.15 to 2.times.10.sup.16 cm.sup.-3, n-type InGaAs doped at 1.times.10.sup.14 to 1.times.10.sup.16 cm.sup.-3, n-type InGaAsP doped at 1.times.10.sup.15 to 1.times.10.sup.16 cm.sup.-3, n-type InP doped at 2.times.10.sup.16 to 4.times.10.sup.16 cm.sup.-3 and n-type InP doped at 1.times.10.sup.15 to 8.times.10.sup.15 cm.sup.-3. In this instance, the buffer layer 12 to the window layer 16 are 1 to 3 .mu.m thick, 1 to 5 .mu.m thick, 0.3 to 1.0 .mu.m thick, 0.8 to 4.0 .mu.m thick and 1 to 2 .mu.m thick, respectively.
The photo-incident region 161 is formed through a diffusion of zinc, and is doped at the 1.times.10.sup.17 to 1.times.10.sup.20 cm.sup.-3. A guard ring 20 is partially overlapped with the photo-incident region 161, and is formed through an ion-implantation of beryllium.
In operation, the p-n junction between the photo-incident region 161 and the multiplication layer 15 is reversely biased so that a depletion layer extends from the p-n junction into the photo-absorbing layer 13. The depleted photo-absorbing layer 13 produces electron-hole pairs in the presence of incidental light at the wavelength not greater than 1.67 microns equivalent to the band-gap of InGaAs, and the carriers are accelerated in the electric field ranging between 20 to 100 kV/cm created in the depleted photo-absorbing layer 13. The carriers reach the saturation velocity by the agency of the strong electric field, and current flows between the electrodes 18 and 19.
The intermediate layer 14 featured in the prior art APD shown in FIG. 2 makes the discontinuity between the valence band of the photo-absorbing layer 13 of InGaAs and the valence band of the multiplication layer 15 of InP smooth. For this reason, the smooth valence band protects the APD from holes liable to be accumulated at the discontinuity, and allows the APD to produce a sharp leading edge of a photo-detecting signal.
The prior art APDs are available for an optical-time-domain-reflectometer (hereinafter abbreviated as OTDR), and the OTDR detects a reflection due to the Rayleigh scattering for a trouble shooting with an optical fiber. If an optical fiber is broken, the light propagated along the optical fiber is reflected at the broken point, and the reflection returns to the entrance of the optical fiber. The OTDR provided at the entrance converts the reflection into an electric pulse signal, and an analyst determines the break point on the basis of a lapse of time between an emission of light pulse and the detected reflection.
The OTDR is coupled with the optical fiber by means of a connector unit, and the connector unit produces a Fresnel diffraction. After a light pulse of, for example, 100 nanometers is emitted, the Fresnel diffraction is firstly detected by the OTDR, and the Rayleigh reflection, thereafter, arrives thereat. In general, the Fresnel diffraction is much stronger than the Rayleigh reflection, and the OTDR can not discriminate the Rayleigh reflection during the Fresnel diffraction and the pulse decay of the electric pulse signal indicative of the Fresnel diffraction. This means that there is a dead zone in the vicinity of the entrance of the optical fiber, and any broken point in the dead zone can not be found by the OTDR.
One of the approaches is to make the trailing edge of the electric pulse signal sharp, and a decreased pulse decay time shrinks the dead zone.
FIG. 3 shows response characteristics of the prior art APDs, and plots Ge and InGaAs are respectively indicative of the response characteristic of the APD of germanium and the response characteristic of the APD in the InP-InGaAs-InGaAsP system.
The APD of germanium is relatively short in decay time rather compared to that of APD in the InP-InGaAs-InGaAsP system. However, the gentle decay of the APD of germanium starts at a relatively large intensity compared to that of the APD of the InP-InGaAs-InGaAsP system.